This invention relates to printing devices. The invention relates more particularly to a means for compensating for printing element flight time and other printer variations which result in columnar misregistration in printers.
In one form of relatively highspeed printing device, a character print head is transported parallel to a stationary platen and its printing elements are selectively activated in a transverse direction toward the platen. The printing elements impact a printing ribbon and a medium which are positioned between the platen and the moving head and forms characters on the medium.
One form of relatively highspeed printer comprises a dot matrix printer. In dot matrix printing, a character is formed by a plurality of printed dots which are selected from a rectangular array or matrix of dot locations arranged in columns and rows. The printing head in a dot matrix printer includes a plurality of individually selectable printing elements or wires which are aligned vertically to form one or more of the dots of a matrix column. These printing wires are accelerated toward the platen by associated solenoids. Scanning of the head along the platen results in the successive columnar printing of additional dots necessary to form the character.
An important consideration in the reproduction of printed characters, particularly when a large quantity of data is being printed, is vertical alignment or registration between characters which extend in a column over a number of horizontally printed lines. Columnar misregistration is undesirable since it detracts from the overall appearance of the printed material. Furthermore, when printing on vertically ruled forms such as may be used for accounting and business purposes, columnar misregistration can result in overlapping of the printed character on a ruled line and obliterate the character or confuse the reader. In dot matrix printing in particular, the overlapping of a character and a vertically ruled line can in some instances result in the alteration of the character.
High speed printers have, at times, exhibited this undesirable columnar misregistration. During the formation of a character by high speed impact printing, each of the printing elements which is advanced along the platen and which has a finite mass exhibits a delay or flight time between a time at which the printing element is initially actuated and a time at which it impacts the print medium to form a character. Since each of the printing elements are moving parallel to the platen, the character will be printed at a location displaced from the point of initial actuation. In dot matrix printers, for example, the printing wires each have a finite mass which incur a delay time between the time when a solenoid energization is initiated and the time a dot is actually formed. The printer includes means for locating the head and for establishing printing locations along the platen. Printing wire flight time results in a divergence or flight time displacement between the print location called for by the printer and the actual position of the printed character.
Highspeed printers often provide for incremental or stepwise printing and for continuous printing. In incremental printing the printing head which is controlled from a keyboard or from a communication line is stepped to a plurality of successively located printing positions at which the character or dot matrix column is printed. During continuous printing the head is maintained in continuous motion and printing occurs while the head is being advanced. The lack of columnation over successive lines often becomes apparent when the printer is operated in an incremental mode and particularly when the input characters are selected from a keyboard. During incremental printing, a flight distance displacement of the printed dots is encountered in the course of each acceleration to, or deceleration from maximum printing velocity. The flight time displacement during acceleration or deceleration can occur over a distance of one to three characters. While in a continuous printing mode, the flight time displacement is not as severe, none the less the highspeed printing technique of printing successive lines in alternately different directions results in a doubling of the lack of columnation which can become apparent.
In addition to the referred-to flight time displacements of the printed dots, other mechanical characteristics of the device contribute to an undesired lack of columnation in the printout. These characteristics include a mechanical cocking of the printhead and the existence of a spring factor in the head drive system. The printhead is generally supported and transported on a track such as, for example, an elongated machined rod which extends in a direction parallel to the platen. In a dot matrix printer, the printhead includes a printhead body which is also accurately machined for transport along the rod. A mechanical tolerance is provided between the track and the transported head to enable binding free start-stop movements. This tolerance permits a cocking or tilting of the printhead on the track when inertial forces occur as the head is being accelerated or decelerated along the track. The cocking is developed between the head and the track and albeit relatively small, it occurs in a direction opposite to the accelerating force and causes a delay in the head movement which contributes to displacement of the printhead character.
In some highspeed printer arrangements, the printhead is accelerated along the track by a drive means which exhibits a spring factor or action. The spring action results in a delay and displacement of the printed character. A spring action occurs, for example, with a drive mechanism utilizing an elastomeric belt.
The flight time, cocking and spring factor displacements both individually and cumulatively contribute to an increased columnar misregistration.
Accordingly, it is an object of this invention to provide an improved printer.
Another object of the invention is to provide an improved printer having means for reducing columnar misregistration.
Another object of the invention is to provide an impact printer having means for compensating for printer characteristics which result in a displacement of printed characters.
Another object of the invention is to provide an improved printer having means for compensating printing element flight time displacement in a printing head.
A further object of the invention is to provide an improved impact printer having means for compensating for printer character displacement resulting from mechanical cocking of the printhead.
Another object of the invention is to provide an improved impact printer having means for compensating for printed character displacement resulting from a spring action characteristic of a print head transport drive.